https://src.fedoraproject.org/rpms/libserf/raw/rawhide/f/libserf-1.3.9-errgetfunc.patch
https://bugs.gentoo.org/805161
--- a/buckets/ssl_buckets.c
+++ b/buckets/ssl_buckets.c
@@ -1204,6 +1204,10 @@
     }
 }
 
+#ifndef ERR_GET_FUNC
+#define ERR_GET_FUNC(ec) (0)
+#endif
+
 static int ssl_need_client_cert(SSL *ssl, X509 **cert, EVP_PKEY **pkey)
 {
     serf_ssl_context_t *ctx = SSL_get_app_data(ssl);